CHICAGO — Junk food remains plentiful at the nation's elementary schools despite widespread efforts to curb childhood obesity, a new study suggests.

Between 2006 and 2010, nearly half of public and private schools surveyed sold sweet or salty snack foods in vending machines or other places, the study found.

There was little change over the four years, a surprising finding given vocal advocacy campaigns to improve kids' diets, said researcher Lindsey Turner, a health psychologist at the University of Illinois at Chicago and the study's lead author.

The study focused on snacks not sold during mealtimes, which until recently weren't subject to government nutrition standards.

Schools most likely to sell chips, cookies or similar foods were in the South, where obesity rates are the highest; these foods were scarcest at schools in the West.

The results are concerning, Turner said, because they show that many schools have not heeded messages from health advocates including the Institute of Medicine, which in a 2007 report urged limiting availability of food in schools outside of mealtimes, and said these items should not be sugary, salty or fatty snack foods.

Many schools in the study also offered more healthy foods outside of mealtimes, including fruit and vege-tables.